Lithographic apparatus and device manufacturing method

ABSTRACT

A lithographic apparatus including: a projection system to project radiation onto a substrate supported on a substrate stage, during an exposure phase; a sensing system to sense a property of the substrate on the stage during a sensing phase; and a positioning system to determine a position of the stage relative to a reference system via a radiation path between the stage and the reference system, wherein the apparatus is configured to control stage movement relative to the reference system in the sensing phase and to control other movement relative to the reference system during the exposure phase; the stage or reference system having an outlet to provide a gas curtain to reduce ingress of ambient gas into the path; and the apparatus is operative such that a characteristic of the gas curtain is different in at least part of the sensing phase compared to in the exposure phase.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the U.S. national phase entry of PCT patentapplication no. PCT/EP2015/073681, which was filed on Oct. 13, 2015,which claims the benefit of EP application no. 14192938.0, which wasfiled on Nov. 13, 2014, each of which are incorporated herein in itsentirety by reference.

FIELD

The present invention relates to an atmospheric lithographic apparatusand a device manufacturing method.

BACKGROUND

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In that instance, a patterning device, whichis alternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.comprising part of, one, or several dies) on a substrate (e.g. a siliconwafer). The substrate may be supported on a substrate stage. Transfer ofthe pattern is typically during an exposure phase via imaging apatterned beam of radiation onto a layer of radiation-sensitive material(resist) provided on the substrate. A projection system is provided forprojecting the patterned beam of radiation onto the target portionduring the exposure phase. In general, a single substrate will contain anetwork of adjacent target portions that are successively patterned.Known lithographic apparatus include so-called steppers. In stepperseach target portion is irradiated by exposing an entire pattern onto thetarget portion at one time. Other known lithographic apparatus includeso-called scanners. In scanners each target portion is irradiated byscanning the pattern through a projection radiation beam in a givendirection (the “scanning”-direction) while synchronously scanning thesubstrate parallel or anti-parallel to this given direction.

Before the exposure phase one or more properties of the substrate is/aresensed during a sensing phase. At least one of the one or moreproperties may be sensed by an alignment system. For example, a surfacetopography of the substrate may be measured during the sensing phase.This process is sometimes known as levelling or levelling scanning.Additionally or alternatively the property which is sensed may be theposition of alignment marks on the substrate relative to other alignmentmarks provided on, e.g., the substrate stage on which the substrate issupported. This process is known as an alignment or alignment scanning.The properties sensed during the sensing phase are used during theexposure phase to ensure correct focus of the patterned beam ofradiation on the substrate and/or correct positioning of the patternedbeam of radiation on the substrate.

In one type of lithographic apparatus the substrate stage for supportingthe substrate is in an environment with an ambient gas. Such alithographic apparatus is called an atmospheric lithographic apparatus.In an atmospheric lithographic apparatus a liquid may be providedbetween the final element of the projection system and the substrateduring the exposure phase. Such an apparatus is often referred to as animmersion lithographic apparatus.

The rate at which the lithographic apparatus applies the desired patternon the substrate, known as throughput, is a major performance criterionin lithography apparatus. A higher throughput is desirable. Throughputis dependent on multiple factors. One factor on which throughput isdependent is the speed at which transfer of the pattern onto thesubstrate takes place. Another factor on which throughput is dependentis the speed at which properties of the substrate which need to besensed prior to transfer of the pattern can be sensed. It is beneficialto have high moving speeds of the substrate during the exposure phaseand/or during the sensing phase. However, it is important to maintainaccuracy of measurements, particularly of determining the position ofthe substrate stage relative to the projection system, alignment systemand/or alignment systems, at the high moving speeds.

Measurement radiation beams are used for determining the position of thesubstrate stage relative to the projection system, alignment systemand/or alignment systems or an intermediate body such as a gridconfigured to cooperate with an encoder system or such as a referenceframe. Measurement radiation beams in atmospheric lithographic apparatuspass through gas along a path of radiation. Local fluctuations in thecharacteristics of gas through which measurement radiation beams passcan affect the measurement radiation beam. Therefore, it is an aim ofthe present invention to provide an apparatus with reduced fluctuationsin the characteristics of gas along the path of radiation.

SUMMARY OF THE INVENTION

According to an aspect of the invention, there is provided anatmospheric lithographic apparatus comprising: a substrate stage forsupporting a substrate in an environment with an ambient gas; aprojection system configured for subsequently projecting a patternedbeam of radiation onto a plurality of target portions of the substrateon the substrate stage during an exposure phase; a sensing system forsensing a property of the substrate on the substrate stage during asensing phase; a reference system; and a positioning system configuredfor determining a position of the substrate stage relative to thereference system via a path of radiation between the substrate stage andthe reference system; wherein: the atmospheric lithographic apparatus isconfigured for controlling the substrate stage to undergo movementrelative to the reference system in the sensing phase and to undergoother movement relative to the reference system during the exposurephase; at least the substrate stage or the reference system has anoutlet system for providing a gas curtain of a barrier gas operative toreduce ingress of the ambient gas into a volume traversed by the path ofradiation between the substrate stage and the reference system; and thelithographic apparatus is operative such that a characteristic of thegas curtain is different in at least part of the sensing phase comparedto in the exposure phase.

According to an aspect of the invention, there is provided a devicemanufacturing method comprising: a sensing phase of sensing a propertyof a substrate on a substrate stage in an environment with an ambientgas; an exposure phase of exposing a pattern from a patterning deviceonto the substrate on the substrate stage; wherein in the sensing phaseand in the exposure phase the position of the substrate stage relativeto a reference system is determined via a path of radiation between thesubstrate stage and the reference system and a gas curtain of a barriergas reduces ingress of the ambient gas into a volume traversed by thepath of radiation between the substrate stage and the reference system;wherein: a characteristic of the gas curtain is different in at leastpart of the sensing phase compared to in the exposure phase.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 depicts a lithographic apparatus according to an embodiment ofthe present invention;

FIG. 2 is a cross-sectional view of a substrate stage of a lithographicapparatus;

FIG. 3 is a plan view of the substrate stage of FIG. 2;

FIG. 4 is a plan view of an outlet system of a further embodiment;

FIG. 5 is two plan views of an outlet system of a further embodiment;

FIG. 6 is plan views of a first layer and a second layer of a furtherembodiment;

FIG. 7 is a cross-sectional view of the outlet system of FIG. 6; and

FIG. 8 shows the two positions of the upper layer and lower layer ofFIGS. 6 and 7.

DETAILED DESCRIPTION

FIG. 1 schematically depicts a lithographic apparatus according to oneembodiment of the invention. The apparatus comprises:

-   -   an illumination system (illuminator) IL configured to condition        a radiation beam B (e.g. UV radiation).    -   a support structure (e.g. a mask table) MT constructed to        support a patterning device (e.g. a mask) MA and connected to a        first positioner PM configured to accurately position the        patterning device in accordance with certain parameters;    -   a substrate stage (e.g. a wafer table) WT constructed to hold a        substrate (e.g. a resist-coated wafer) W and connected to a        second positioner PW configured to accurately position the        substrate in accordance with certain parameters; and    -   a projection system (e.g. a refractive projection lens system)        PS configured to project a patterned beam of radiation (formed        when a pattern is imparted to the radiation beam B by patterning        device MA) onto a target portion C (e.g. comprising one or more        dies) of the substrate W.

The atmospheric lithographic apparatus comprises an enclosure EN. Theenclosure EN encloses at least the substrate stage WT. An ambient gas ispresent in the enclosure EN.

The illumination system IL may include various types of opticalcomponents, such as refractive components, reflective components,magnetic components, electromagnetic components, electrostaticcomponents or other types of optical components, or any combinationthereof, for directing radiation, shaping radiation, or controllingradiation.

The support structure MT supports, i.e. bears the weight of, thepatterning device MA. The support structure MT holds the patterningdevice MA in a manner that depends on the orientation of the patterningdevice MA, the design of the lithographic apparatus, and otherconditions, such as for example whether or not the patterning device MAis held in a vacuum environment. The support structure MT may be a frameor a table, for example, which may be fixed or movable as required. Thesupport structure MT may ensure that the patterning device MA is at adesired position, for example with respect to the projection system PS.Any use of the terms “reticle” or “mask” herein may be consideredsynonymous with the more general term “patterning device.”

The term “patterning device” used herein should be broadly interpretedas referring to any device that can be used to impart a projectionradiation beam with a pattern in its cross-section such as to create apattern in a target portion C of the substrate W. It should be notedthat the pattern imparted to the projection radiation beam may notexactly correspond to the desired pattern in the target portion C of thesubstrate W, for example if the pattern includes phase-shifting featuresor so called assist features. Generally, the pattern imparted to theradiation beam B to form the patterned beam of radiation will correspondto a particular functional layer in a device being created in the targetportion C, such as an integrated circuit.

The lithographic apparatus may be of a type having two (dual stage) ormore substrate stages WT (and/or two or more mask tables MT). In such“multiple stage” machines the additional substrate stage(s) WT and/ormask table(s) MT may be used in parallel. Alternatively preparatorysteps may be carried out on one or more substrate stage(s) WT and/ormask table(s) MT while one or more other substrate stage(s) WT and/ormask table(s) MT are being used for transfer of the pattern onto thesubstrate W.

The lithographic apparatus may also be of a type wherein at least aportion of the substrate W may be covered by an immersion liquid havinga relatively high refractive index, e.g. water, so as to fill a spacebetween the projection system PS and the substrate W. Immersiontechniques are well known in the art for increasing the numericalaperture of projection system PS. The term “immersion” as used hereindoes not mean that a structure, such as a substrate, must be submergedin immersion liquid, but rather only means that immersion liquid islocated between the projection system PS and the substrate W duringtransfer of the pattern onto the substrate.

During an exposure phase, the radiation beam B is incident on thepatterning device (e.g., mask MA), which is held on the supportstructure (e.g., mask table MT), and is patterned by the patterningdevice to form a patterned beam of radiation. Having traversed the maskMA, the patterned beam of radiation passes through the projection systemPS, which projects the patterned beam of radiation onto a target portionC of the substrate W. With the aid of the second positioner PW andposition sensor IF (e.g. a linear encoder with grid G as illustrated inFIG. 1), the substrate stage WT can be moved accurately, e.g. so as toposition different target portions C in the path of the patterned beamof radiation. Similarly, the first positioner PM and another positionsensor (which is not explicitly depicted in FIG. 1) can be used toaccurately position the mask MA with respect to the path of thepatterned beam of radiation, e.g. after mechanical retrieval from a masklibrary, or during a scan in a scanner. In general, movement of the masktable MT may be realized with the aid of a long-stroke module (coarsepositioning) and a short-stroke module (fine positioning), which formpart of the first positioner PM. Similarly, movement of the substratestage WT may be realized using a long-stroke module and a short-strokemodule, which form part of the second positioner PW. In the case of astepper (as opposed to a scanner) the mask table MT may be connected toa short-stroke actuator only, or may be fixed. Mask MA and substrate Wmay be aligned using mask alignment marks M1, M2 and substrate alignmentmarks P1, P2. Although the substrate alignment marks P1, P2 asillustrated occupy dedicated target portions C, they may be located inspaces between target portions C (these are known as scribe-lanealignment marks). Similarly, in situations in which more than one die isprovided on the mask MA, the mask alignment marks P1, P2 may be locatedbetween the dies.

The depicted lithographic apparatus could be used in a scanning mode,i.e. as a scanner. In the scanning mode, the mask table MT and thesubstrate stage WT are scanned synchronously while the patterned beam ofradiation is projected onto a target portion C (i.e. a single dynamicexposure). The velocity and direction of the substrate stage WT relativeto the mask table MT may be determined by the (de-)magnification andimage reversal characteristics of the projection system PS.

Before the exposure phase one or more properties of the substrate is/aresensed during a sensing phase. The surface topography of the substratemay be measured (often called leveling or leveling scanning) during thesensing phase. Additionally or alternatively the position of alignmentmarks on the substrate W relative to alignment marks on the substratestage WT may be measured (often called alignment or alignment scanning)during the sensing phase. The properties sensed during the sensing phaseare used during the exposure phase to ensure correct focus of thepatterned beam of radiation on the substrate W and/or correctpositioning of the patterned beam of radiation on the substrate W.

During the exposure phase and the sensing phase the position of thesubstrate stage WT relative to a reference system RF is determined. Apositioning system is provided for determining the position of thesubstrate stage WT relative to the reference system RF. The positioningsystem determines the position of the substrate stage WT relative to thereference system RF via a path of radiation between the substrate stageWT and the reference system RF. By knowing (i) the position of thesubstrate stage WT relative to the reference system RF, (ii) theposition of the reference system RF relative to the projection systemPS, and (iii) the position of the substrate W relative to the substratestage WT, the position of the substrate W relative to the patterned beamof radiation may be determined.

An embodiment of a positioning system for determining the position ofthe substrate stage WT relative to the reference system RF is depictedin FIGS. 2 and 3. FIGS. 2 and 3 are a side view and a plan viewrespectively of a substrate stage WT. The positioning system comprises aradiation source 20 for emitting a measurement radiation beam 50, and asensor 40 for detecting the measurement radiation beam 50. Themeasurement radiation beam 50 is projected towards a grid G of thereference system RF. In this embodiment the position of the grid Grelative to the projection system PS is known. In the embodimentillustrated, the reference system RF may be implemented by a referenceframe, indicated here by the same acronym RF. The position of the grid Grelative to the reference frame RF is known and may or may not be fixed.The relative position of the reference frame RF to the projection systemPS is known. The position of the reference frame RF relative to theprojection system PS may or may not be fixed. In this way the grid G isin a known position relative to the projection system PS. Themeasurement radiation beam 50 is reflected and/or refracted by the gridG back to the sensor 40. The measurement radiation beam 50 travels alonga path of radiation. The sensor 40, configured to detect the measurementradiation beam 50, is used to indicate the position and/or movement ofthe grid G relative to the radiation source 20 and/or the sensor 40. Thesensor 40 measures displacements of the substrate stage WT relative tothe grid G. Therefore, the position of the substrate stage WT relativeto the projection system PS can be determined. This is possible becausethe position of the grid G relative to the projection system PS is knownas described above.

A respective combination of a radiation source 20 and a sensor 40 ismost conveniently positioned at a respective one of the corners of thesubstrate stage WT, see e.g., the diagram of FIG. 3, explained furtherbelow. This convenient position is due to the center of the substratestage WT being taken up by the substrate W. Furthermore, pairs ofdiametrically positioned combinations are involved in determining anangular displacement of the substrate stage WT about an axis parallel tothe Z axis (see the diagram of FIG. 1). The angular displacement can bedetermined with higher accuracy if the distance between thediametrically positioned combinations is larger. For more background,see, e.g., U.S. Pat. No. 7,602,489 issued to Van der Pasch et al., andincorporated herein by reference.

As described above, the positioning system uses a measurement radiationbeam 50. The measurement radiation beam 50 travels along the path ofradiation. Ambient gas through which the measurement radiation beam 50passes along the path of radiation may affect the measurement radiationbeam 50.

Several factors can affect how the measurement radiation beam 50propagates through a gas. For example, temperature of the gas, humidityof the gas and composition of the gas are factors which may affect therefractive index of a gas. Localised variations of these factors andturbulence in the gas can result in non-uniformities in the refractiveindex of the gas. The measurement radiation beam 50 passing through agas is affected by variations in the refractive index. For example, achange in the refractive index can alter the trajectory of themeasurement radiation beam 50. Additionally or alternatively, a changein the refractive index can introduce wavefront errors into themeasurement radiation beam 50. Measurement errors can be induced byvariations in the refractive index along the path of radiation.Measurement errors can lead to positioning inaccuracies in thepositioning of components of the lithograph apparatus. Any suchpositioning inaccuracies can alter the placement of the patterned beamof radiation on the substrate W and so can have a detrimental effect onoverlay and/or focus.

Known arrangements are in place to try to reduce the fluctuations in therefractive index of gas in a volume traversed by the path of radiation.For example, in an embodiment an outlet system 3 is provided. The outletsystem 3 is configured to provide a gas curtain 13 of a barrier gasoperative to reduce ingress of the ambient gas in the enclosure EN intothe volume traversed by the path of radiation. Therefore, gas in thevolume through which the measurement radiation beam 50 passes can becontrolled.

A known outlet system 3 ejects barrier gas from at least one opening 30in a surface of the substrate stage WT. The barrier gas forms a gascurtain 13 which impedes the flow of ambient gas on one side of the gascurtain 13. A gas curtain 13 can be provided around the volume such thatthe gas within the volume is effectively separated from the ambient gasoutside the volume. The gas within the volume can be conditioned suchthat it is more uniform than the gas outside of the volume. Therefore,the gas curtain 13 can be used to provide a barrier around the volumetraversed by the path of radiation of the measurement radiation beam 50.This protects the measurement radiation beam 50 from the effects ofchanges in ambient gas outside the volume. The gas within the volume isreferred to as the protected gas.

Any unconditioned ambient gas which enters into the volume can affectthe propagation of the measurement radiation beam 50 and induce errors.Outlet systems 3 use various different ways of preventing ambient gasfrom entering the volume using gas curtains 13. The different waysinclude, but are not limited to (i) providing a jet of barrier gasthrough a single set of openings 30 in the substrate stage WT and (ii)providing a turbulent flow of barrier gas through a first set ofradially inward openings in the substrate stage WT surrounding thevolume radially inwardly with respect to the volume of a laminar flow ofthermally conditioned barrier gas provided through a second set ofradially outward openings in the substrate stage WT. For completeness,it is remarked here that the flow of the barrier gas through the firstset of radially inward openings may be turbulent or laminar.

However, tests on a known outlet system 3 have shown that as movingspeed increases, more and more unconditioned ambient gas enters into thevolume and contaminates the protected gas.

During relative movement of the substrate stage WT in the ambient gas ofthe enclosure EN, a flow of the ambient gas relative to the substratestage WT is induced as follows. Movement of the substrate stage WT inthe enclosure EN causes the ambient gas to be pushed out of the way ofthe substrate stage WT at a side of the substrate stage WT acting as thefront side of the substrate stage WT during the movement. This pushingaway creates an increase in pressure of the ambient gas at the frontside of the substrate stage WT. The movement also causes a decrease inpressure of the ambient gas at a side of the substrate stage WT actingas a back side of the substrate stage WT during the movement. Thedifference in pressure of the ambient gas between the front side of thesubstrate stage WT and the back side of the substrate stage WT causes aflow of ambient gas from the front side to the back side of thesubstrate stage WT.

Any flow of ambient gas over the substrate stage WT imposes an inwardsforce on the gas curtain 13. The inwards force on the gas curtain 13increases with increased velocity of the flow of ambient gas. Thevelocity of the flow of ambient gas increases with increasing velocityof the substrate stage WT relative to the enclosure EN. As the inwardsforce increases, ambient gas from outside the volume traversed by thepath of radiation is forced into the volume. Gas entering into thevolume in this way can be referred to as break-through.

At high moving speeds and/or at long movement durations, break-throughof thermally unconditioned ambient gas into the volume can besignificant. High moving speeds are desirable to increase throughput.

The present invention aims to reduce break-through whilst limiting theacoustic disturbances generated by the outlet system 3. The presentinvention is based on the insight that the type of movements performedby the substrate stage WT during different phases of operation of theatmospheric lithographic apparatus varies. For example, during thesensing phase, the substrate stage WT typically makes moves at highervelocities relative to the reference frame RF compared to during theexposure phase. A higher velocity of the substrate stage WT relative tothe reference frame RF increases the chance of break-through. During thesensing phase the length of movements between changes in directionrelative to the reference frame RF are much longer than during theexposure phase. The inventors have found that this gives any ambient gasflow more time to create sufficient instability in the gas curtain 13thereby to achieve break-through. Additionally the directions ofmovement of the substrate stage WT relative to the reference frame RFare predominantly in one direction in the sensing phase and are moreevenly distributed between different directions in the exposure phase.

Existing outlet systems 3 are optimised for the case where high constantvelocities of the substrate stage WT relative to the reference frame RFoccur for an infinitely long duration of movement in a given direction.To withstand such high constant velocities, high velocities of barriergas relative to the outlet system 3 when exiting the outlet system 3 areused to create the gas curtain 13. High velocities of barrier gasrelative to the outlet system 3 undesirably create acoustic disturbancesin the apparatus. Acoustic disturbances are lower at lower barrier gasvelocities relative to the outlet system 3.

In the present invention, different characteristics of the gas curtain13 are used in at least part of the sensing phase compared to in theexposure phase. The different characteristics of the gas curtain 13 areoptimised for the exposure phase and the at least part of the sensingphase to resist break-through whilst using as low as possible a velocityof barrier gas, thereby to reduce acoustic disturbances.

In an embodiment, during the transition between the sensing phase andthe exposure phase, the characteristics of the gas curtain 13 arechanged from one characteristic to another.

In further embodiments, the characteristics of the gas curtain 13 may bedifferent during other phases or sub-phases of operation of thelithographic apparatus. An example of another phase is a moving phaseduring which the substrate stage WT moves between a position at whichthe sensing phase takes place and a position at which the exposure phasetakes place. Examples of sub-phases of operation are levelling scanningand alignment scanning which are sub-phases of the sensing phase. Usinga different gas curtain 13 characteristic between the levelling scanningand alignment scanning may be advantageous. This is because during analignment scan the maximum speed of the substrate stage WT relative tothe reference frame RF is different to (e.g., lower than) the maximumspeed of the substrate stage WT relative to the reference frame RFduring a levelling scan. In an embodiment, characteristics of the gascurtain 13 are the same for the exposure phase and the alignmentscanning sub-phase of the scanning phase, but different for thelevelling scanning sub-phase of the scanning phase.

The present inventors envisage two or more different characteristics ofgas curtain 13 for different phases or sub-phase of operation of thelithographic apparatus.

In the present invention operating characteristics of the outlet system3 (i.e., characteristics of the gas curtain 13) are varied according tothe types of movement expected during the current phase of operation.The characteristics which are varied include: (i) the speed of thebarrier gas relative to the outlet system 3 when exiting the outletsystem 3, (ii) a volume of the barrier gas exiting the outlet system 3per unit of time, and (iii) a spatial distribution, in plan, of thebarrier gas when exiting the outlet system 3. In the case that theoutflow area of the outlet system 3 does not change, (i) and (iii) areeffectively the same. Varying the characteristic allows a reduction inthe speed of the barrier gas relative to the outlet system 3 whenexiting the outlet system 3 at least during (i) at least part of thesensing phase (e.g., during a sub-phase) and (ii) the exposure phase,compared to prior art machines. As a result acoustic disturbancesresulting from the high speed of the barrier gas relative to the outletsystem 3 when exiting the outlet system 3 are reduced during at leastpart of the sensing phase and/or during the exposure phase compared toprior art machines.

For movements which are more likely to result in break-through (highvelocity and/or long distance between changes in direction), a higherbarrier gas velocity exiting the outlet system 3 is used. At lowervelocities and/or distances between changes in direction of movement ofthe substrate stage WT, lower barrier gas velocity exiting the outletsystem 3 is used, thereby reducing acoustic disturbance.

A difference between the movements of the substrate stage WT relative tothe reference system during at least part of the sensing phase andexposure phase is the predominate direction of travel relative to thereference frame RF of the substrate stage WT. Therefore, in the casethat the geometry (e.g., spatial distribution) in plan of the gascurtain 13 formed by the barrier gas is not circular, differentgeometries and/or orientations of the gas curtain 13 may be more suitedto one of the sensing phase or one of its sub-phases and the exposurephase than the other.

Several embodiments of outlet system 3 with outlet system controller 100will now be described. Each of the embodiments may vary one or more of:the speed of the barrier gas relative to the outlet system 3 whenexiting the outlet system 3, the volume of barrier gas exiting theoutlet system 3 per unit of time, and the spatial distribution of thebarrier gas when exiting the outlet system 3.

The invention is described below with reference to FIG. 2 in which thesubstrate stage WT accommodates at least one outlet system 3. Thesubstrate stage WT is illustrated at an imaging location under theprojection system PS. However, in an embodiment the at least one outletsystem 3 may instead be part of the reference system RF and mountedsubstantially stationary relative to the projection system PS (e.g.,accommodated on the reference frame RF) along with the radiation source20 and sensor 40. In that embodiment, the grid G is not part of thereference system but moves with substrate stage WT and is in knownposition relative to the substrate stage WT (e.g., fixed to thesubstrate stage WT).

In an embodiment, a grid G which is part of the reference system isadditionally at a measurement location. Properties of a substrate Wmounted on the substrate stage WT such as position of the substrate W onthe substrate stage WT, surface topography of the substrate W, etc., aremeasured at the measurement location. In this embodiment the grid G maybe positioned above the substrate stage WT (similar to the mainembodiment described above) or may be positioned on the substrate stageWT as described in the preceding paragraph.

On the substrate stage WT of FIG. 3, four outlet systems 3 can be seen.Other objects may be included on the substrate table WT which have notbeen shown, for example an object configured to hold a substrate W. Eachoutlet system 3 is configured to provide a gas curtain 13 operative toreduce an inflow of ambient gas into a volume traversed by the path ofradiation between the substrate stage WT and the reference system RF.Each of the outlet systems 3 shown comprises at least one opening 30 inthe substrate stage WT. The at least one opening 30 in the substratestage WT is adapted for a flow of barrier gas therefrom for establishingthe gas curtain 13 enclosing part of the volume traversed by the path ofradiation.

FIG. 3 illustrates a first embodiment of outlet system 3. A plurality ofindividual openings 30 in an upper surface of the substrate stage WTsurrounds the radiation source 20 and sensor 40. A controller 100individually controls the flow of barrier gas out of each of theopenings 30 of the outlet system 3. The controller 100 controls thespeed of barrier gas exiting each of the openings 30 relative to therespective opening 30. The speed of barrier gas relative to therespective opening 30 may be the same for each of the plurality ofopenings 30. In an alternative embodiment the speed of the barrier gasrelative to the respective opening 30 may vary between openings 30. Forinstance, any of openings 30 aligned with the radiation source 20 and/orsensor 40 in the principle directions of movement of the substrate stageWT may have a higher gas flow rate out of them than other openings 30.The controller 100 changes the speed of barrier gas relative to therespective opening 30 during the exposure phase compared to at leastpart of the sensing phase (e.g., the levelling scanning sub-phase).Because (i) the speed of the substrate stage WT relative to thereference system RF is lower during the exposure phase than during thepart of the sensing phase, and (ii) the time between the changes indirection of movement of the substrate stage WT is lower during theexposure phase than during the at least part of the sensing phase (e.g.,the levelling scanning sub-phase), the controller 100 decreases thespeed of barrier gas exiting each of the openings 30 during the exposurephase compared to the part of the sensing phase (e.g., the levellingscanning sub-phase). As a result, any acoustic disturbances generated bythe barrier gas are lower during the exposure phase than would be thecase if the speed of barrier gas exiting each of the openings 30relative to the respective opening 30 were not lower during the exposurephase.

Although the embodiment of FIG. 3 shows plural openings 30 per outletsystem 3, a single elongate opening may be present instead.

The embodiment of FIG. 3 has been described above with reference to thespeed relative to the opening 30 when exiting the outlet system 3.However, in an alternative embodiment, the controller 100 may insteadcontrol the volume of barrier gas exiting the openings 30 per unit oftime.

FIG. 4 illustrates a further embodiment which is the same as the FIG. 3embodiment except as described below. In the embodiment of FIG. 4, theoutlet system 3 comprises a first elongate opening 210 and a secondelongate opening 230. The controller 100 controls a valve 220 whichselects whether barrier gas is provided to the first elongate opening210 or the second elongate opening 230.

Each of the first elongate opening 210 and second elongate opening 230surround the radiation source 20 and sensor 40. Each of the firstelongate opening 210 and second elongate opening 230 may be provided asa groove or may be formed by a plurality of individual openings.

The geometry of the first elongate opening 210 is configured for useduring the exposure phase. The geometry of the first elongate opening210 is configured for a movement of the substrate stage WT which doesnot have a predominate direction of travel. That is, the geometry of thefirst elongate opening 210 is configured to provide a gas curtain 13with good resistance to break-through during travel of the substratestage WT in any horizontal direction (i.e., in the x y plane).

The geometry of the second elongate opening 230 is configured for useduring at least part of the sensing phase (e.g., during the levellingscanning sub-phase). The geometry of the second elongate opening 230 isconfigured to provide a gas curtain 13 which is especially resistant tobreak-through during movement in the left and right direction asillustrated. This is the predominant direction of movement of thesubstrate stage WT during at least part of the sensing phase (e.g.,during the levelling scanning sub-phase). Therefore, for a given speedof barrier gas relative to the outlet system 3 when exiting the outletsystem 3, the geometry of the second elongate opening 230 provideshigher resistance to break-through during movement to the right and leftas illustrated than the geometry of the first elongate opening 210.

As in the embodiment of FIG. 3 and all other embodiments, the controller100 may vary the speed of the barrier gas relative to the outlet system3 when exiting the outlet system 3 and/or the volume of barrier gasexiting the outlet system 3 per unit of time. The controller 100 mayvary the speed and/or volume depending on whether in the exposure phaseor in a certain part of the sensing phase (e.g., the levelling scanningsub-phase).

FIG. 5 illustrates, in plan, a further embodiment of an outlet system 3.The embodiment of FIG. 5 is the same as that of FIG. 3 except asdescribed below. The concept is similar to that of the FIG. 4 embodimentin that the barrier gas is provided through openings with a differentgeometry in at least part of the sensing phase (e.g. in the levellingscanning sub-phase) to the geometry of the openings through which thebarrier gas is provided in the exposure phase (and optionally during theremaining part of the sensing phase).

A plurality of elongate openings are provided. The controller 100controls to which of the plurality of openings barrier gas is supplied.An inner opening 310 and an outer opening 320 are provided. The inneropening 310 has a shape, in plan, of a square. The outer opening 320 isin the form of two V shapes on their side pointing away from theradiation source 20 and sensor 40 in the directions of predominatetravel of the substrate stage WT during at least part of the sensingphase (e.g., during the levelling scanning sub-phase). Four flowdirectors 330 are provided, one at each corner of the inner opening 310(though only one is illustrated in FIG. 5 for improved clarity). Uponreorientation of the flow directors 330 under control of the controller100, a first outlet geometry or second outlet geometry can be selected.In the left hand side of FIG. 5, the first outlet geometry which haspointed sides is illustrated as being selected which is chosen for useduring the sensing phase. The first geometry is made up of the outeropenings 320 and top and bottom parts of the inner opening 310. On theright hand side of FIG. 5 the flow directors 330 are re-orientated toselect the second outlet geometry which has a substantially squareshape. The second outlet geometry is comprised of all of the inneropening 310 and none of the outer opening 320. The second outletgeometry of the right hand side of FIG. 5 is used in the exposure phase(and optionally during part of the sensing phase). Each of the flowdirectors 330 are re-orientated during at least part of the sensingphase (e.g., during the levelling scanning sub-phase) so as to shift thegeometry from the second outlet geometry to the first outlet geometry.

FIG. 6 shows an upper layer 410 and lower layer 420 of a furtherembodiment of an outlet system 3, in plan. FIG. 7 shows the embodimentof FIG. 6 in cross-section and FIG. 8 shows the upper layer 410 andlower layer 420 one on top of another in plan. The embodiment of FIGS.6-8 is similar to that of FIG. 5 in that the geometry of the openingsthrough which barrier gas is supplied can be changed between a firstoutlet geometry for use during at least part of the sensing phase (e.g.,during the levelling scanning sub-phase) and a second outlet geometryfor use during the exposure phase.

In the embodiment of FIGS. 6-8 an upper layer 410 and a lower layer 420are provided one on top of the other. Movement of the upper layer 410relative to the lower layer 420 changes the geometry of the openingsthrough which barrier gas is provided. A translational motion of theupper layer 410 relative to the lower layer 420 between the exposurephase and at least part of the sensing phase (e.g. the levellingscanning sub-phase) allows for a change in outlet geometry.

The lower layer 420 is shown on the left hand side of FIG. 6. Openingsof the first geometry and of the second geometry are present, bothcentred with respect to their position relative to the radiation source20 and sensor 40.

The right hand side of FIG. 6 shows the upper layer 410. Openings of thefirst outlet geometry and of the second outlet geometry are provided.However, the first outlet geometry and second outlet geometry are offset from one another with respect to the radiation source 20 and sensor40. When the upper layer 410 and lower layer 420 are placed one on topof another, and barrier gas is provided at pressure underneath them, bymoving the upper layer 410 relative to the lower layer 420 it ispossible to select through which parts of the openings in the upperlayer 410 barrier gas is allowed to flow.

A lithographic apparatus in accordance with at least one of the aboveembodiments can be used in a device manufacturing method to irradiate asubstrate using a projection radiation beam.

In an embodiment, there is provided an atmospheric lithographicapparatus comprising: a substrate stage for supporting a substrate in anenvironment with an ambient gas; a projection system configured forsubsequently projecting a patterned beam of radiation onto a pluralityof target portions of the substrate on the substrate stage during anexposure phase; a sensing system for sensing a property of the substrateon the substrate stage during a sensing phase; a reference system; and apositioning system configured for determining a position of thesubstrate stage relative to the reference system via a path of radiationbetween the substrate stage and the reference system; wherein: theatmospheric lithographic apparatus is configured for controlling thesubstrate stage to undergo movement relative to the reference system inthe sensing phase and to undergo other movement relative to thereference system during the exposure phase; at least the substrate stageor the reference system has an outlet system for providing a gas curtainof a barrier gas operative to reduce ingress of the ambient gas into avolume traversed by the path of radiation between the substrate stageand the reference system; and the lithographic apparatus is operativesuch that a characteristic of the gas curtain is different in at leastpart of the sensing phase compared to in the exposure phase.

In an embodiment, the characteristic comprises at least one of thefollowing: a speed of the barrier gas relative to the outlet system whenexiting the outlet system; a volume of the barrier gas exiting theoutlet system per unit of time; and a spatial distribution of thebarrier gas when exiting the outlet system.

In an embodiment, there is provided a device manufacturing methodcomprising: a sensing phase of sensing a property of a substrate on asubstrate stage in an environment with an ambient gas; an exposure phaseof exposing a pattern from a patterning device onto the substrate on thesubstrate stage; wherein in the sensing phase and in the exposure phasethe position of the substrate stage relative to a reference system isdetermined via a path of radiation between the substrate stage and thereference system and a gas curtain of a barrier gas reduces ingress ofthe ambient gas into a volume traversed by the path of radiation betweenthe substrate stage and the reference system; wherein a characteristicof the gas curtain is different in at least part of the sensing phasecompared to in the exposure phase.

In an embodiment, the characteristic comprises at least one of thefollowing: a speed of the barrier gas relative to the substrate stage; avolume of the barrier gas exiting an outlet system for providing the gascurtain per unit of time; and a spatial distribution of the barrier gasrelative to the substrate stage.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,flat-panel displays, liquid-crystal displays (LCDs), thin-film magneticheads, etc. The skilled artisan will appreciate that, in the context ofsuch alternative applications, any use of the terms “wafer” or “die”herein may be considered as synonymous with the more general terms“substrate” or “target portion”, respectively. The substrate referred toherein may be processed, before or after exposure, in for example atrack (a tool that typically applies a layer of resist to a substrateand develops the exposed resist), a metrology tool and/or an inspectiontool. Where applicable, the disclosure herein may be applied to such andother substrate processing tools. Further, the substrate may beprocessed more than once, for example in order to create a multi-layerIC, so that the term substrate used herein may also refer to a substratethat already contains one or more processed layers.

Although specific reference may have been made above to the use ofembodiments of the invention in the context of optical lithography, itwill be appreciated that the invention may be used in otherapplications, for example imprint lithography, and where the contextallows, is not limited to optical lithography. In imprint lithography atopography in a patterning device defines the pattern created on asubstrate. The topography of the patterning device may be pressed into alayer of resist supplied to the substrate whereupon the resist is curedby applying electromagnetic radiation, heat, pressure or a combinationthereof. The patterning device is moved out of the resist leaving apattern in it after the resist is cured.

The invention has been explained in this text and in the accompanyingdiagrams with reference to a specific configuration of a lithographicapparatus, which has a grid G at a reference system RF and which hasrespective combinations of a respective radiation source 20 and of arespective sensor 40 accommodated at the substrate stage WT. Theinvention is likewise applicable to another configuration of alithographic apparatus, which has a grid G at the substrate stage WT andwhich has respective combinations of a respective radiation source 20and of a respective sensor 40 accommodated at the reference system RF.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.having a wavelength of or about 436, 405, 365, 355, 248, 193, 157 or 126nm), as well as particle beams, such as ion beams or electron beams.

The term “lens”, where the context allows, may refer to any one orcombination of various types of optical components, includingrefractive, reflective, magnetic, electromagnetic and electrostaticoptical components.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described.

The descriptions above are intended to be illustrative, not limiting.Thus, it will be apparent to one skilled in the art that modificationsmay be made to the invention as described without departing from thescope of the claims set out below.

The invention claimed is:
 1. A lithographic apparatus comprising: asubstrate stage configured to support a substrate in an environment withan ambient gas; a projection system configured to project a beam ofradiation onto the substrate when supported on the substrate stageduring an exposure phase; a sensing system configured to sense aproperty of the substrate on the substrate stage during a sensing phase;a reference system; and a positioning system configured to determine aposition of the substrate stage relative to the reference system via apath of radiation between the substrate stage and the reference system;wherein: the lithographic apparatus is configured to control thesubstrate stage to undergo movement relative to the reference system inthe sensing phase and to undergo other movement relative to thereference system during the exposure phase; at least the substrate stageor the reference system has an outlet system configured to provide a gascurtain of a barrier gas operative to reduce ingress of the ambient gasinto a volume traversed by the path of radiation between the substratestage and the reference system; and the lithographic apparatus isconfigured to provide a characteristic of the gas curtain to bedifferent in at least part of the sensing phase compared to in theexposure phase, wherein the characteristic comprises a spatialdistribution of the barrier gas when exiting the outlet system and thelithographic apparatus is configured to provide the spatial distributionof the barrier gas when exiting the outlet system to be different in atleast part of the sensing phase compared to in the exposure phase. 2.The lithographic apparatus of claim 1, wherein the characteristicfurther comprises at least one selected from: a speed of the barrier gasrelative to the outlet system when exiting the outlet system; and/or avolume of the barrier gas exiting the outlet system per unit of time. 3.The lithographic apparatus of claim 1, wherein the outlet system isconfigured to provide a gas curtain having a different geometry aroundthe path during at least part of the sensing phase than in the exposurephase.
 4. The lithographic apparatus of claim 3, wherein the gas curtainduring at least part of sensing phase is created using a first openingof the outlet system or using a first combination of openings of theoutlet system and the gas curtain in the exposure phase is created usinga second opening of the outlet system different than the first openingor using a second combination of openings of the outlet system differentthan the first combination of openings.
 5. The lithographic apparatus ofclaim 3, wherein the geometry of the gas curtain during at least part ofthe sensing phase has a dimension approximately equal in two orthogonaldirections and the geometry of the gas curtain in the exposure phase hasdifferent dimensions in two orthogonal directions.
 6. The lithographicapparatus of claim 1, wherein the lithographic apparatus is configuredto provide the spatial distribution of the barrier gas when exiting theoutlet system to be different in at least part of the sensing phasecompared to in the exposure phase such that an area spanned by the gascurtain is different in at least part of the first phase compared to inthe second phase.
 7. The lithographic apparatus of claim 1, wherein thelithographic apparatus is configured to provide a shape of the gascurtain forms a pointed shape with a point thereof pointing in a majordirection of movement of the substrate stage.
 8. A device manufacturingmethod comprising: a sensing phase of sensing a property of a substrateon a substrate stage in an environment with an ambient gas; an exposurephase of exposing a pattern from a patterning device onto the substrateon the substrate stage; wherein in the sensing phase and in the exposurephase the position of the substrate stage relative to a reference systemis determined via a path of radiation between the substrate stage andthe reference system and a gas curtain of a barrier gas reduces ingressof the ambient gas into a volume traversed by the path of radiationbetween the substrate stage and the reference system, wherein acharacteristic of the gas curtain is different in at least part of thesensing phase compared to in the exposure phase, and wherein thecharacteristic comprises a spatial distribution of the barrier gas whenexiting the outlet system and the spatial distribution of the barriergas when exiting the outlet system is different in at least part of thesensing phase compared to in the exposure phase.
 9. The devicemanufacturing method of claim 8, wherein the characteristic furthercomprises at least one selected from: a speed of the barrier gasrelative to the substrate stage; and/or a volume of the barrier gasexiting an outlet system for providing the gas curtain per unit of time.10. The device manufacturing method of claim 8, wherein the gas curtainhas a different geometry around the path during at least part of thesensing phase than in the exposure phase.
 11. The device manufacturingmethod of claim 10, wherein the geometry of the gas curtain during atleast part of the sensing phase has a dimension approximately equal intwo orthogonal directions and the geometry of the gas curtain in theexposure phase has different dimensions in two orthogonal directions.12. The method of claim 8, wherein the spatial distribution of thebarrier gas when exiting the outlet system is different in at least partof the sensing phase compared to in the exposure phase such that an areaspanned by the gas curtain is different in at least part of the firstphase compared to in the second phase.
 13. The method of claim 8,wherein a shape of the gas curtain forms a pointed shape with a pointthereof pointing in a major direction of movement of the substratestage.
 14. A lithographic apparatus comprising: a substrate stageconfigured to support a substrate in an environment with an ambient gas;a projection system configured to project a beam of radiation onto thesubstrate when supported on the substrate stage during an exposurephase; a sensing system configured to sense a property of the substrateon the substrate stage during a sensing phase; a reference system; and apositioning system configured to determine a position of the substratestage relative to the reference system via a path of radiation betweenthe substrate stage and the reference system; wherein: the lithographicapparatus is configured to control the substrate stage to undergomovement relative to the reference system in a first phase and toundergo other movement relative to the reference system during a secondphase; at least the substrate stage or the reference system has anoutlet system configured to provide a gas curtain of a barrier gasoperative to reduce ingress of the ambient gas into a volume traversedby the path of radiation between the substrate stage and the referencesystem; and the lithographic apparatus is configured to provide acharacteristic of the gas curtain to be different in at least part ofthe first phase compared to in the second phase, wherein thecharacteristic comprises an area spanned by the gas curtain and thelithographic apparatus is configured to provide the area spanned by thegas curtain to be different in at least part of the first phase comparedto in the second phase.
 15. The lithographic apparatus of claim 14,wherein the first phase is a first sub-phase of the sensing phase andthe second phase is a second sub-phase of the sensing phase.
 16. Thelithographic apparatus of claim 14, wherein the lithographic apparatusis operative such that a characteristic of the gas curtain is differentin at least part of the exposure phase compared to in the first and/orsecond phase.
 17. The lithographic apparatus of claim 14, wherein thecharacteristic further comprises at least one selected from: a speed ofthe barrier gas relative to the outlet system when exiting the outletsystem; and/or a volume of the barrier gas exiting the outlet system perunit of time.
 18. The lithographic apparatus of claim 14, wherein theoutlet system is configured to provide a gas curtain having a differentgeometry around the path during at least part of the sensing phase thanin the exposure phase.
 19. The lithographic apparatus of claim 18,wherein the gas curtain during at least part of sensing phase is createdusing a first opening of the outlet system or using a first combinationof openings of the outlet system and the gas curtain in the exposurephase is created using a second opening of the outlet system differentthan the first opening or using a second combination of openings of theoutlet system different than the first combination of openings.
 20. Thelithographic apparatus of claim 14, wherein the lithographic apparatusis configured to provide a shape of the gas curtain that forms a pointedshape with a point thereof pointing in a major direction of movement ofthe substrate stage.
 21. The lithographic apparatus of claim 14, whereinthe lithographic apparatus is configured to provide a geometry of thegas curtain during at least part of the sensing phase having a dimensionapproximately equal in two orthogonal directions and a geometry of thegas curtain in the exposure phase having different dimensions in twoorthogonal directions.